Abstract

An earthquake swarm was observed in the Blue Mountain Lake area of the southern Adirondacks from early May 1971 until the writing of this paper (late April, 1972). This swarm provided an unusual opportunity for a detailed field study of earthquakes and a determination of principal stress direction for a region within a lithospheric plate. Thousands of events were recorded, the largest of which had a magnitude of 3.6 and was felt as far as 80 km from the epicenter. A number of earthquakes were heard as well as felt—the smaller were heard, but not felt.

Thrusting mechanisms were determined from two composite fault-plane solutions. The solution for earthquakes above 2-km depth indicates faulting on a plane striking N12°W and dipping 25°E. The fault plane for the deeper events (between 2 and 3.5 km) strikes N31°E and dips 59°E.

The earthquake foci define a surface that dips gently to the east to a depth of 2 km and then steepens, in agreement with the composite fault-plane solutions. There is an indication that the shallower earthquakes may not represent renewed motion on a pre-existing fault, but may mark the generation of a new fault by a regional east-west compressive stress. The deeper events which occurred late in the sequence may represent the extension of the shallow fault and its deflection to an existing weakness. The axis of maximum compressional principal stress for the shallow composite fault-plane solution trends approximately east-west and is nearly horizontal. Geological and geophysical evidence is presented to support the hypothesis that the greatest principal stress in a zone extending from northern New York State to southern Illinois is compressive, large and horizontal, and trends nearly east-west.

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